The present invention relates to a method and a circuit arrangement, respectively, for digitally regulating the carrier phase error in receivers of digital data transmission systems of the type wherein a comparator is used to determine, from the sampled value of the demodulated input signal and from the estimated value for each sampling time associated therewith by means of a decider, the deviation dk which is a measure for the phase difference between the sampled value and the estimated value. This deviation value is then filtered in a digital loop filter, including a proportional branch and an integration branch, and the regulating value is obtained in a subsequently connected accumulator.
Such a method and circuit arrangements for practicing the method are disclosed in the paper by Kammeyer and Schenk, "Ein Flexibles Experimentier-system fur die Datenubertragung im Fernsprechbereich" (A Flexible Experimental System for Data Transmission in the Telephone Field), FREQUENZ (Frequency), 1979, Volume 33, No. 5, pages 141-145 and No. 6, pages 165-172 as well as in an article by Gockler, Hofmeister and Till "Datenubertragung mit 4.8 kBit/s im Fernsprechnetz" (Data Transmission with 4.8 kBit/s in a Telephone Network) WISSENSCHAFTLICHE BERICHTE (Scientific Reports) AEG-TELEFUNKEN, 1978, Vol. 51, No. 4/5. pages 231-245. In this described system, the phase information for the carrier regulation is derived from the input and output signals of the decider. The decider input signal can be represented in the signal plane by a vector which for quadrature amplitude modulation (QAM) includes the components xk and yk, these components being sampled values of the analytical signal after demodulation. In the decider, these sampled values are associated with estimated values ak, bk. The average of the phase difference between the actual value vector and the desired value vector corresponds to the carrier phase error. By means of the carrier phase error regulation, the demodulation phase is caused to be regulated until the phase difference on the average becomes zero. For this purpose, a deviation dk is determined in a comparator for each sampling time k.multidot.T as it results from the corresponding linkage of the input and output signals of the decider. After filtering of the deviation value dk in a digital loop filter with subsequent accumulation, there results the angle of additional rotation of the signal vector as required for coherent demodulation. The digital loop filter includes a proportional branch and an integration branch. Both branches of the filter can be weighted by means of respective multipliers or multiplying values ki or kv, respectively. However, even with favorable dimensioning of these two weighting factors it is possible that in the nontransient final state other parameter settings occur which are of similar quality criteria but are otherwise unusable.